HOW BIOLOGIST CLASSIFY ORGANISMS *Biologist classify organisms based on appearance, structure, breeding patterns, and similarities in DNA sequence.

I. What is a Species? species – a group of natural populations that are interbreeding reproductive isolation – occurs when two or more groups of organisms can no longer interbreed; theoretically, two different species cannot interbreed with one another

In nature, however, reproductive barriers between sexually reproducing species are not always complete. Sometimes individuals of different species interbreed and produce offspring called hybrids. (ex: dogs-wolves) Many species of plants, some mammals, and many fish are able to form fertile hybrids with one or few closely related species.

Dog-Wolf Hybrid Wolves and dogs can produce fertile offspring. Genus

Donkey and Horse Hybrid The offspring of a donkey and a horse is a mule. The mule, however, is sterile. Donkey: Equus asinus Horse: Equus caballus

II. Evolutionary Histories A. Phylogeny – the evolutionary history of an organism Phylogenies can be shown as branching tree diagrams called phylogentic trees. B. Cladistics is a classification system based on common ancestry; it reconstructs phylogenies by inferring relationships based on shared characters Cladistics can be shown using a branching diagram called a cladogram.

A cladogram is a branching, treelike diagram in which the endpoints of the branches represent specific species of organisms.

A node is the intersection of two branches. A node represents the most recent common ancestor.

A clade is a group of organisms that share certain traits derived from a common ancestor. A group that has none of the characters is known as an out-group.

FYI: The taxa group (organism) that has ALL the characters will be at the top of the cladogram and the taxa group with the least characters will be at the bottom of the cladogram. Remember, the most distantly related organism is called an out-group.

Cladogram: What organism is most closely related to the bird? What organisms have the derived character of hair? What ancestral character is common to ALL the organisms? Crocodile Primates, rodents, rabbits vertebrae 4. Circle the node that represents a common ancestor of the primates, rodents & rabbits, crocodiles, birds?

Phylogenetic Tree A node is the intersection of two branches. A node represents the most recent common ancestor.

Phylogenetic Tree: 1. Which organisms are most closely related? a. cow and camel b. camel and goat c. minke whale and cow 2. Which organisms are least a. cow and sheep b. sheep and minke whale c. camel and sheep 4 5 3 2 4 5

Two different ways of representing the same pattern of evolutionary histories: Cladogram Phylogenetic Tree